Ischemia is an acute condition associated with an inadequate flow of oxygenated blood to a part of the body, caused by the constriction or blockage of the blood vessels supplying it. This reduction in blood flow can result from the blockage of a vessel by an embolus (blood clot), the blockage of a vessel due to atherosclerosis, etc. The restriction of blood flow quickly causes necrosis in the effected tissues.